1. Field of the Invention
The present invention relates to a cable modem system and, more particularly, to a cable modem system for performing data communication by utilizing cable television (CATV) transmission path.
2. Description of the Related Art
In the cable television network, broadcasting signals are transmitted from a CATV station to each subscriber via a trunk line and a branch line. The trunk line may be a multistage tree type or a multistage star type. Since the trunk line and the branch lines are used to transmit the wide-band broadcasting signals, coaxial cables and optical fiber cables are chiefly used for the trunk line and the branch lines. Accordingly, the CATV system has a superior wide-band transmission ability.
Recently, Internet has been rapidly widespread. As a result, it is concerned to the broad-band transmission ability with an excellent CATV system. The preparation to use CATV transmission path as an access network of Internet is being advanced. Especially, standardization is being advanced by using CATV transmission path as for the cable modem system to connect the subscriber to Internet by a LAN interface. The telco return method and the 2-way method have been standardized.
In the 2-way method, the CATV transmission path is used for both the down-link to transmit signals from the CATV station to the subscriber side and the up-link to transmit signals from the subscriber side to the CATV station. Detail of the specification of the 2-way method is described in xe2x80x9cData-Over-Cable Service Interface Specifications, Radio Frequency Interface Specification SP-RFI-I04-980724xe2x80x9d.
In the telco return method, the CATV transmission path is used for the down-link and the public switch telephone network (PSTN) is used for the up-link. Detail of the specification of the telco return method is described in xe2x80x9cData-Over-Cable Service Interface Specifications, Cable Modem Telephony Return Interface Specification SP-CMTRI-I01-970804.
A description will now be given, with reference to FIGS. 1 and 2, of the 2-way method and the telco return method. FIG. 1 is a structural diagram of a cable modem system using the 2-way method. FIG. 2 is a structural diagram of a cable modem system using the telco return method.
The cable modem system shown in FIG. 1 comprises a CATV station 10, a CATV transmission path 20 and a subscriber unit 30. The CATV station 10 comprises a cable modem termination system (CMTS) 11 connected to the CATV transmission path 20 and a router 14 connected to Internet.
The CATV transmission path 20 includes a coaxial cable and an amplifier for amplifying attenuated RF signals. An optical fiber cable may be used together with the coaxial cable depending on the transmission distance. The subscriber unit 30 includes a cable modem (CM) 31 and a terminal unit 35 such as a personal computer which supports a standard Internet protocol TCP/IP. The terminal unit 35 is connected to the CATV transmission path 20 via the cable modem 31.
When the subscriber connects the terminal unit 35 to Internet, the Internet protocol (IP) packet output from the terminal unit 35 is supplied to a LAN end terminal 33 of the cable modem 31 via the LAN. The cable modem 31 converts the supplied IP packet into a multiple frame of the time division multiplexer access (TDMA) method. Thereafter, the multiple frame is digitally modulated to an RF signal by an up-link RF end terminal 32, and the RF signal is output to the CATV transmission path 20.
The RF signal output to the CATV transmission path 20 is supplied to the cable modem termination system 11 of the CATV station 10. The RF signal is then digitally demodulated by an up-link RF end terminal 12, and the multiple frame is disassembled so as to reproduce the original IP packet. The cable modem termination system 11 sends the original IP packet to Internet via the router 14.
On the other hand, an IP packet supplied from Internet is supplied to the cable modem termination system 11 via the router 14. The cable modem termination system 11 converts the supplied IP packet into a multiple frame of a time division multiplex (TDM) method. The multiple frame is digitally modulated to an RF signal by a down-link RF end terminal 13, and is output to the CATV transmission path 20. The RF signal output to the CATV transmission path 20 is supplied to the cable modem 31 of the subscriber. The RF signal is digitally demodulated by a down-link RF end terminal 34, and the multiple frame is disassembled so as to reproduce the original IP packet. The original IP packet is supplied to the terminal unit 35 via the LAN.
The cable modem system using the telco return method shown in FIG. 2 comprises the CATV station 10, the CATV transmission path 20, the subscriber 30 and a public service telephone network (PSTN) 40. In FIG. 2, parts that are the same as the parts shown in FIG. 1 are given the same reference numerals, and descriptions thereof will be omitted.
The CATV station 10 includes the cable modem termination system 11, the router 41 and a telco remote access concentrator (TRAC) 15 which is connected to the public service telephone network 40.
When the subscriber connects the terminal unit 35 to Internet, the cable modem 31 is activated by turning on a power switch. The down-link RF end terminal 34 of the cable modem 31 searches for an RF signal which is supplied from the CATV station 10 via the CATV transmission path 20 which serves as a down-link to the CATV center 10 so as to receive a dial number, a user name and a password that are necessary for the connection to Internet.
Then, a telco return end terminal 37 included in the cable modem 31 sends a call to the public service telephone network 40 by using the received dial number so as to establish a communication line to a line end functional unit 16 of the telco remote access concentrator 15.
After the communication line is established, a link is established in accordance with the point-to-point protocol (PPP) which is a standard Internet protocol for a serial connection. At this time, the user name and the password received by the down-link RF end terminal 34 of the cable modem 31 are used for attestation. By this attestation, it can be determined whether or not the subscriber is an authorized user by determining whether or not the user name has been registered.
Thereafter, the IP packet output from the terminal unit 35 is supplied to the LAN end terminal 33 of the cable modem 31 via the LAN. The IP packet is converted into an encapsulated signal in accordance with the point-to-point protocol by the telco return protocol unit 17. The encapsulated signal is supplied to a telco return end terminal 37, and is modulated to a modem signal of V.34. The modem signal is supplied to the telco remote access concentrator 15 via the public service telephone network 40.
The line end functional unit 16 of the telco remote access concentrator 15 demodulates the supplied modem signal. Thereafter, the telco return protocol unit 17 removes the encapsulation by the protocol so as to reproduce the original IP packet. The telco remote access concentrator 15 sends the reproduced IP packet to Internet via the cable modem termination system 11 and the router 14. On the other hand, the IP packet supplied by Internet is supplied to the terminal unit 35 via the CATV transmission path 20 in the same manner as that of the 2-way method.
The telco return method described with reference to FIG. 2 uses a public service telephone network for the up-link. However, a public personal handyphone system (PHS) network 50 may be used as shown in FIG. 3, or a private PHS network may be used as shown in FIG. 4.
The cable modem system using the telco return method shown in FIG. 3 uses the public PHS network for the up-link. Accordingly, the CATV station 10 is provided with a terminal adapter (TA) 18 which is compatible with the PHS Internet access forum standard (PIAFS), and the subscriber 30 is provided with a personal station (PS) 38 which is compatible with the PIAFS. Additionally, the public PHS network 50 is provided with a cell station (CS) 51 corresponding to the personal station 38.
The cable modem system using the telco return method shown in FIG. 4 uses the private PHS network for the up-link. Accordingly, the CATV station 10 is provided with a private branch exchange (PBX) 19 compatible with PHS, an RF modem 21 and a mixer 22. The subscriber is provided with the personal station 38 compatible with the PIAFS, a cell station 52 compatible with the PIAFS and an RF modem 53. Additionally, the cable modem system using the telco return method shown in FIG. 4 uses a part of the CATV transmission path 20 for the up-link. Accordingly, the RF modems 21 and 53 are provided between the cell station 52 and the private branch exchange 19.
Since the cable modem system using the 2-way method shown in FIG. 1 uses the CATV transmission path 20 for the up-link, there is a problem related to the up-link noise which is inherent to the CATV system. In the CATV system, since the CATV transmission path 20 is constructed as a multistage tree type or a multistage star type, the up-link noises entering the subscriber path to the CATV transmission path 20 are added and gathered to the CATV station 10. The up-link noise has a bandwidth corresponding to the up-link which is 10 MHz to 55 MHz in Japan and 5 MHz to 42 MHz in the United States.
Additionally, the bandwidth of the up-link is easily affected by a noise such as an electronic equipment noise in the subscriber 30, an engine spark noise of an automobile or an armature radio communication noise. Thus, when the up-link noises reach the CATV station 20, there may be a bandwidth which is not usable due to a high-level noise. Accordingly, a part of the up-link of which bandwidth is narrower than that of the down-link must be reserved for a bandwidth exclusive for the cable modem. This may be a problem in view of an efficient use of the bandwidth. Additionally, there is a problem in that a signal is destroyed by the up-link noise.
Further, in a case in which the cable modem system using the 2-way method is used in housing complex such as an apartment house, there is a problem in that a source of the up-link noise cannot be specified and, thus, it is difficult to prevent such a noise from entering the CATV transmission path 20. This is because the source of the noise does not always present in the subscriber""s house. Additionally, in many cases, the mutual agreement of the entire resident is necessary for the housing complex to change the CATV transmission path to a bidirectional path.
On the other hand, according to the cable modem system using the telco return method shown in FIGS. 2 or 3, there may be no problem if the telephone cost is low. However, if the telephone cost is high and charged to the subscriber, there is a problem in that this cable modem system cannot consist as a business.
Additionally, the cable modem system using the telco return method using the private PHS network shown in FIG. 4 is provided with the RF modem between the cell station 52 which constitutes the up-link and the private branch exchange 19. Accordingly, a bandwidth must be reserved for the RF modems 21 and 53 in the bandwidth of the CATV transmission path 20 when the terminal unit 35 is not performing communication. This is a problem in view of an efficient use of the bandwidth. Especially, since a part of the bandwidth of the up-link which bandwidth is narrower than that of the down-link must be reserved exclusively for the cable modem, and considering the bandwidth lower than 30 MHz is not usable due to the up-link noise, there is a problem in that a number of cell stations 52 is limited.
It is a general object of the present invention to provide a cable modem system in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a cable modem system in which an up-link noise is prevented from entering a cable so as to efficiently use the bandwidth of the CATV transmission path.
It is another object of the present invention to provide a cable modem system which can reduce an initial cost charged to a subscriber.
A further object of the present invention is to provide a cable modem system which can reduce a communication cost charged to a subscriber so as to spread the cable modem system.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a cable modem system performing data communication using a transmission path of a cable television system which includes a cable television station apparatus and a plurality of subscriber units connected to each other by the transmission path, the cable modem system comprising:
a gateway apparatus receiving an original data signal supplied by one of the subscriber units via a telephone network, the gateway apparatus converting the original data signal into a converted data signal so that the converted data signal complies with a protocol corresponding to a destination address included in the original data signal, the gateway apparatus sending the converted data signal to the cable television station apparatus via the transmission path; and
a line end terminal unit, provided in the cable television station apparatus, receiving the converted data signal sent from the gateway apparatus via the transmission path, the line end terminal unit restoring the converted data signal to the original data signal.
In the cable modem system according to the present invention, the subscriber unit can operate based on the telco return method and the CATV station can operate based on the 2-way method. Thus, the up link noise can, be reduced. Additionally, since only a part of the CATV transmission path between the CATV station and the gateway apparatus should be changed to a bidirectional, a construction is not needed at the subscriber unit side.
Additionally, by locating the gateway apparatus at a position adjacent to the subscriber unit within a distance at which the PHS radio communication can be performed, an up link can be established on the subscriber unit side without providing an additional cable line to the subscriber unit. Additionally, the telco return line used for the up link can be provided by the CATV service supplier who provides the cable modem service, a communication cost charged to the subscriber can be reduced.
Further, by providing a gate keeper to the CATV station, the dial number and the IP address can be unitarily managed, which facilitates connection to a plurality of destination. Additionally, the contents of a table provided in each gateway apparatus which table stores information regarding dial numbers and IP addresses can be easily changed or updated.
Additionally, by providing an H.323 function unit, which supports a function according to the ITU-T recommendation H.323, each of the gateway apparatus and the CATV station, various services including the Internet connection service according to a dial up method or the telephone service using the personal handyphone system (PHS) can be provided in addition to the Internet connection service using the cable modem.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.